(a) Field of the Invention
The present invention relates to a toner concentration sensor and, more particularly, to a toner concentration sensor for use in a PPC copying machine or an electrostatic printer using a wet-development electrostatic photographic technique.
(b) Description of Related Art
In general, wet-development-type electrostatic photographic system such as PPC copying machine and electrostatic printer uses a liquid developer wherein a toner including pigments and resins as main components thereof is dispersed in a carrier solvent including aliphatic hydrocarbons obtained from petroleum. Examples of the aliphatic hydrocarbon solvent used therein include Isoper-E, -G, -H, -L, -K and -M and Noper 12 from Exxon Co., and Shellzol 71 and Solvesso 150 from Shell Oil Group. Examples of the pigments used therein include carbon black, azo pigments and multi-cyclic pigments.
In an electrostatic photographic machine (ESPM) using such a liquid developer it is known that the toner concentration in the liquid developer reduces with the proceeding of the recording operation, thereby degrading the image density. Thus, it is important in the ESPM to maintain the toner concentration at a constant in the liquid developer by replenishing the toner therein while detecting the toner concentration. The toner concentration in the liquid developer is detected by a variety of toner concentration sensors each optically detecting the toner concentration.
FIG. 1 shows the principle of a conventional toner concentration sensor, generally designated by numeral 15. The toner concentration sensor 15 includes a housing 11 disposed in a suitable location of the ESPM for defining a passage of the liquid developer 10, a pair of optical members 12 having a gap therebetween and disposed within the housing 11 for penetrating the same, and a combination of a light emitting device 13 and a photodetector 14 optically coupled together by the optical members 12.
The light emitting device 13 emits light through the optical members 12 when the liquid developer 10 is introduced in the housing 11. The emitted light passes through the liquid developer 10 at the gap between both the optical members 12 to be detected by the photodetector 14. The reduction of the toner concentration allows the light intensity detected by the photodetector 14 to increase. The ESPM replenishes the toner into the liquid developer 10 to thereby maintain the toner concentration in the liquid developer at a constant.
The optical members 12 in the toner concentration sensor 15 should have properties such as higher durability against the liquid developer and higher transparency for the light. The optical member 12 is generally made of glass or plastic materials in view of having the above properties. However, as shown in FIG. 2, the surface of some of the optical members 12 have a large number of hydroxyl groups 16 thereon. Such an optical member 12 has a large surface energy depending on the polarity of the hydroxyl groups and a large surface energy of the hydrogen bonds, and the adsorption of the components in the liquid developer, such as the pigments and charge control agents, onto the hydroxyl group 16 accelerates contamination of the optical member 12.
In addition, since the carrier solvent of the liquid developer including the aliphatic hydrocarbons obtained from petroleum generally has no polarity, a higher interaction acts between the toner and the optical member depending on the polarity and the surface energy of the hydrogen bonds to thereby accelerate the contamination of the optical member. The contamination reduces the transparency of the optical member, whereby the ESPM erroneously controls the liquid developer to have a lower toner concentration. As a result, the image density is reduced.
JP Utility Model Application No. 63-040351 describes a toner concentration sensor having a cleaning mechanism for cleaning the emitting surface and the receiving surface of the optical members. However, the cleaning mechanism raises the cost for the toner concentration sensor and also increases the size thereof.
Patent Publication JP-A-51-11454 describes a toner concentration sensor having an optical member coated with a film for prevention of surface contamination caused by the physical adsorption of the toner. The film includes a mixture of siloxane polymer having branch, cyclic, multi-cylic or mesh structures and a compound having Si-N bonds. The described structure, however, achieves a lower degree for prevention of contamination, or a lower anti-fouling function, because the surface energy is not sufficiently reduced in the toner including pigments having a higher cohesive property and a higher physical adsorption property in the case of the adsorption property mainly affected by a dispersion force.